System and method for synthetic vision terrain display

ABSTRACT

A synthetic rendering of the terrain within a selected field of view is created from raw terrain elevation data, and the resolution of the rendering is proportioned to the altitude above ground level (altitude AGL) of the aircraft. All the data points are subdivided into tiles. Only data from tiles within a prescribed field of view are considered for processing; all others are ignored. Within the selected tiles, only some of the terrain elevation data points are passed to the graphics processor for rendering. At maximum resolution, when the aircraft is on the ground or at a low altitude, there are relatively fewer tiles within the field of view and the fraction of the data points from each tile within the field of view passed for rendering is relatively large. As the aircraft&#39;s altitude AGL level increases, the field of view also increases in area increasing the number of tiles within the field of view. However, the number of data points forwarded for rendering by the graphics processor is kept approximately constant by selecting progressively fewer and fewer terrain elevation data points from each tile for rendering as altitude AGL increases. Only the data points that lie within tiles inside a prescribed field of view are forwarded to the graphics processor for rendering. Once the tiles have been selected, the terrain elevation data points are grouped in groups of a size that depends on altitude AGL, with more terrain data points in the grouped as attitude increases. Preferably the groups are triangle strips. Only the highest elevation from each group is passed to the graphics processor for rendering.

This application claims the benefit of U.S. Provisional Application No.60/303,578 filed Jul. 6, 2001.

FIELD OF THE INVENTION

The present invention addresses the problem of presenting an appropriatesynthetic image of the terrain below an aircraft to the pilot on ascreen such as a CRT or flat panel display.

BACKGROUND OF THE INVENTION

Contemporary aircraft make extensive use of computer generated displays.Compared to earlier instrumentation, computer generated displays areeasier for pilots to use and to understand; an advantage that can proveimportant when quick decisions must be made. One portion of such adisplay could be a synthetic view of the ground below the aircraft. Sucha view can be generated from raw terrain data such as the commerciallyavailable U.S. Geological Survey Digital Elevation Model data or theDefense Mapping Agency's Digital Terrain Elevation Data.

Raw terrain data is stored in a large table. The table storesinformation about the elevation of each location within the geographicboundaries covered by the table. When an X and a Y coordinate arespecified, the system returns an elevation for that particular pointfrom the table. Data points may be, for example, 300 feet apart making atable covering, for example, the United States, extremely large. Such atable would be too large to present in complete detail on a cockpitdisplay. The huge number of data points would simply overwhelm knownprocessors. Even presenting a small section of such a table, such as thedata points corresponding to all that is in the field of view of a pilotflying at 10,000 feet could present computational and graphic displaydifficulties.

SUMMARY OF THE INVENTION

The present invention provides a method and system for rendering animage of terrain for display in a cockpit display, wherein a set oftiles is formed from raw terrain data, each tile including informationconcerning the elevation of points on the ground. A subset of tiles isselected to be processed to render the image, and the selected subset oftiles is sent to a graphics processor to render an image based on theinformation in the tiles.

More particularly, a preferred method according to the invention createsa synthetic rendering of the terrain within a selected field of viewbelow an aircraft. The method forms tiles from raw elevation terraindata, with each tile including information concerning the elevation ofat least one point, and preferably many points, on the ground. Each tileincludes terrain elevation data for a fixed geographic region. A subsetof the tiles is selected for processing by a graphics processor thatrenders the information included in the selected tiles as athree-dimensional image. The method also proportions the resolution ofthe rendering to the altitude above ground level (altitude AGL) of theaircraft. At maximum resolution, when the aircraft is on or near theground, some or all of the data points from the terrain elevation datathat are within the field of view are forwarded to a graphics processorfor rendering. As the aircraft's altitude AGL increases, the field ofview also increases in area, but the number of data points forwarded forrendering by the graphics processor is kept approximately constant. Thisis accomplished by grouping the data points within each tile intogroups. Representative elevations from each group are passed to thegraphics processor for rendering. At low altitudes only a few tiles arewithin field of view, and the system directs that each tile besubdivided into a relatively large number of groups. As altitudeincreases, the number of tiles that are within the field of viewincreases, and the number of groups into which each tile is subdivideddecreases. As a result, the total number of terrain elevation datapoints that are sent to the graphics processor remains approximatelyconstant. When selecting terrain elevation data points from within agroup for rendering, the sets of adjacent data points are reviewed todetermine which ones to send to the processor.

According to another aspect of the invention, there is provided a methodand system for rendering an image of terrain for display in a cockpitdisplay of an aircraft, characterized by selecting from a set of datapoints representing information concerning the elevation of pointscorresponding to the terrain being traversed by the aircraft, a subsetof data points that are processed for display on the cockpit display,wherein the number of data points per unit area of terrain beingdisplayed is decreased with increasing altitude over ground level whilethe area of terrain corresponding to the selected data points isincreased with increasing altitude over ground level, and vice versa.

The foregoing and other features of the invention are hereinafter fullydescribed and particularly pointed out in the claims, the followingdescription and the annexed drawings setting forth in detail one or moreillustrative embodiments of the invention, such being indicative,however, of but one or a few of the various ways in which the principlesof the invention may be employed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cockpit display showing a synthetic rendering ofthe ground beneath an aircraft.

FIG. 2 is a schematic illustration of a computer system suitable forcarrying out the present invention.

FIG. 3 is a flow diagram illustrating the process of creating tiles fromraw terrain data for use by a graphics processor.

FIG. 4 shows schematically an aircraft and portions of the terrain thatare to be rendered as part of the display of FIG. 1 and portions thatare to be culled.

FIG. 5 is a flow diagram illustrating the process of selecting tiles forrendering by a graphics processor.

FIG. 6 is a flow diagram illustrating the process for rendering tiles bya graphics processor.

DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention provides for the production of a synthetic imageof terrain below and in front of an aircraft, e.g., the image 10 shownin FIG. 1. The image produced is adjusted to conform to a selected fieldof view and to the immediacy of objects of concern. A system 12 forcarrying out the method of the present invention is shown schematicallyin FIG. 2. The system 12 includes a computer 14 (which may be a generalpurpose computer or a dedicated, specially designed computer), a memory16 containing raw terrain elevation data, a graphics processor 18 and adisplay 20. The display 20 may be, for example, a cathode ray tube(CRT), a liquid crystal display screen, a gas plasma-based flat paneldisplay, or other suitable display device.

The process of rendering an image from the raw terrain data takes placein two broad steps. First, a set of tiles representing terrain elevationdata for specific geographic regions is created. Then, the set of tilesis addressed and the pertinent information from selected tiles is passedto the graphics processor to render the image.

The instructions for carrying out the present invention may be stored inany recordable medium such as a hard disk drive, magnetically recordabletape, a compact disk, or as written instructions on paper. They may bestored in the memory 16. The memory 16 may include both volatile andnonvolatile memory components. Volatile components are those that do notretain data values upon loss of power. Nonvolatile components are thosethat retain data upon a loss of power. Thus, the memory 16 may comprise,for example, random access memory (RAM), read-only memory (ROM), harddisk drives, floppy disks accessed via an associated floppy disk drive,compact discs accessed via a compact disc drive, magnetic tapes accessedvia an appropriate tape drive, and/or other memory components, or acombination of any two or more of these memory components. In addition,the RAM may comprise, for example, static random access memory (SRAM),dynamic random access memory (DRAM), or magnetic random access memory(MRAM) and other such devices. The ROM may comprise, for example, aprogrammable read-only memory (PROM), an erasable programmable read-onlymemory (EPROM), an electrically erasable programmable read-only memory(EEPROM), compact flash memory, or other like memory device.

The first step in carrying out a preferred process according to thepresent invention is to prepare terrain elevation data from raw,commercially (or otherwise) available terrain elevation data. This isdone by transforming the terrain altitude data from a format that has ahierarchical structure of quadrangles each of which includes a subset ofdata points into a database in which each data point can be addresseddirectly without any hierarchical structure. This transformation may bedone off line or it may be done on the fly in the computer 14. Theresulting database consists of terrain elevation data, in which eachgeographic location has associated with it an elevation, in particularan elevation above mean sea level. These elevations are known as “posts”or “elevation posts”, and they correspond to elevations above mean sealevel for evenly spaced locations on the earth's surface. If the terrainelevation database is prepared off line, it is loaded into the memory16. If it is created on the fly as required, the raw terrain elevationdata is stored in memory 16 and prepared as required.

Preferred logic 28 for preparing terrain tiles used in carrying out thepresent invention is shown in FIG. 3. The first step 30 (FIG. 3) ingenerating data for rendering a display is to divide the terrainelevation data into tiles. For convenience the terrain elevation datapoints may be organized into conventional triangle strips, a processshown at 32 in FIG. 3. These strips are then grouped into tiles each ofwhich covers a specific fraction of a conventional one degreequadrangle.

A conventional one degree quadrangle covers a square about 60 nauticalmiles by 60 nautical miles at the equator, though the area is less athigher latitudes. In the commercially available databases, elevationpoints are generally spaced in an array about 1200×1200 uniformly acrossthe quadrangle. At latitudes above 75 degrees, the commerciallyavailable databases may supply data in other formats such as 600×1200.Without regard to the initial format, square tiles (although other tileshapes may be used) are formed from the raw terrain data. The tiles areformed by subdividing each side of each quadrangle by a pre-selectednumber, preferably three or four. When each side is divided into three,each quadrangle is divided into in 9 tiles. Each side of each tile thenhas 400 data points that are about 300 feet apart (at full resolution).When divided into a three by three grid, the resulting data points arearrayed in a uniform grid 400×400 in each tile.

This specification proceeds using a 400×400 matrix for each tile witheach elevation post about 300 feet apart. But it should be understoodthat 300×300 or other sizes of terrain tile matrices as well as otherelevation post spacings are possible.

In addition to dividing the raw terrain data into tiles, the computercalculates the geometric center of each tile (36, FIG. 3). All the tilestogether with all the elevation posts in them and their geometriccenters are stored for later use (38, FIG. 3).

Once the terrain elevation data has been converted into a set of tiles,the tiles to be rendered are selected based on the aircraft's positionand heading. As shown in FIG. 4, this is done by selecting a field ofview (FOV) angle 48 to the left and right of the aircraft's trueheading, which is considered the pilot's line of sight (LOS). While theparticular field of view angle may be varied, lines 50 and 52 each at 45degrees to the aircraft's true heading 56 provide a workable example. Asnoted above each tile has associated therewith a geometric center. Indetermining which tiles will be rendered, the geometric centers arecompared to the field of view. Any tile with a geometric center outsidethe field of view angle 48 is rejected and not processed further,according to the preferred embodiment.

Next, the horizon 54 is determined. The distance to the horizon 54 is afunction of the aircraft's altitude above ground level (altitude AGL).While on the ground, the horizon is closest. For a pilot sitting 10 feetabove the ground the horizon is typically about four nautical miles on aclear day. Tiles within the specified angle but beyond the horizon areculled and not further processed at this time. All the culled tiles areshaded in FIG. 4.

The system controls the resolution of the rendering of the terrain ininverse proportion to the aircraft's altitude AGL. Once the tiles to berendered have been determined, the resolution is determined by selectingthe number of data points from each tile that are to be rendered (80)(FIG. 6). Once this is accomplished (as described below), representativeelevation post data from each tile are passed to the graphics processor18 for rendering on the display 20.

On the ground, it has proven sufficient if the terrain data points areconsidered in groups of five (5). As the altitude increases, the terraindata points are considered in larger groups, perhaps as large as onehundred (100) or more. Within each group of terrain elevation datapoints, the elevation values of a representative few are only passed tothe graphics processor. Those few may be selected in a variety of ways,but it has proven practical to select the terrain elevation data pointsthat represent the highest elevation from among its near neighbors forrendering as representative of the entire group (82). First the methodis easy to practice, and second it is conservative in that it makes theterrain appear higher than it may really be on average, but this leavesthe pilot with too much ground clearance, not too little which could bethe consequence of other selection criteria.

The number of terrain elevation data points in each group determines theresolution of the image 10 (FIG. 1) displayed. The lower the number ofelevation posts in a group the higher the resolution. It would be mostpreferable if the initial resolution had only a single terrain elevationdatum point per group. However, that amount of data might unacceptablyslow down the graphics processor 18. Accordingly, there is a trade offbetween graphics processor speed and resolution. An initial grouping ofthe terrain elevation data points into groups of 2, 3, 5, or 8 may benecessary in order to accommodate the graphics processor. Thesegroupings would result in ½, ¼, ⅕ or ⅛, respectively, of all the terrainelevation data points being passed to the graphics processor, with acommensurate reduction in the demand placed on it.

While there are a number of methods for grouping the elevation posts andselecting the representive ones from each group, the triangle stripsformed in step 32 provide a method for doing so. As noted above, it hasbeen found that adequate resolution is obtained when the elevation postsare grouped in groups of five or fewer, where the posts are 300 feetapart (actually 303.97 feet apart). Since every flight must begin on theground, initially the elevation posts are considered in groups of five,four, two or one, depending on the processing capacity of the graphicsprocessor 18.

To divide a 400×400 array of terrain elevation posts into groups, farexample, of five, the data points from the database are set out in asquare grid (84). The grid is subdivided into strips of adjacent rightisosceles triangles that have each of their equal legs parallel to oneside of the grid and their hypotenuses parallel to a diagonal linethrough the grid (86). Each of the equal legs is five elevation postslong, and as a result every fifth data paint lies at one of the verticesof a triangle (88). The triangles completely cover the tile.

While strips of triangles have been described, other ways of subdividingthe tiles could be used. For example, the tiles could be subdivided intosquares or hexagons or any regular combination of polygons thatcompletely covers the tile surface.

The elevation posts at the vertices of the triangles are initiallyselected to be passed to the graphics processor for rendering asrepresentative of the entire triangle. However, in order to assure thatno elevation post that is taller than the elevation posts at thevertices is missed as is preferred, the elevation of the elevation postat each vertex is compared to the elevation of elevation posts around it(90). If there is an elevation post within the immediate region (to bediscussed below) that is taller than the elevation post at the vertex,then the elevation post at the vertex is reassigned the higher elevationvalue (92). Only after this comparison has been made and the vertex hasbeen reassigned as its value the elevation of the highest nearbyelevation post, is the height of the data post at each vertex passed tothe graphics processor for rendering (94).

The height of the elevation post at a vertex should be compared withenough neighbors to assure that all the elevation posts are included ina comparison with at least one nearby vertex. Where a triangle's equallegs are five elevation posts long, the vertices should be compared toevery data post that is within three data posts of the vertex. In thisway every elevation post is considered in at least one comparisonoperation.

While one process of selecting a representative value for each vertexelevation post has been disclosed, others are possible. For example, nostructure or geographic feature rises higher than 14,500 above mean sealevel in the continental U.S. So, within the continental U.S., once thataltitude is reached, the vertex posts could be used directly withoutcomparison to its neighbor. Other methods of assigning values to theelevation posts at the vertexes of the triangles will occur to thoseskilled in the art and may be used without varying from the spirit ofthis invention.

In order to assure that every elevation post is considered, ispreferable that the tiles be subdivided into an integer number ofgroups. Where the tiles include 160,000 data points (400 by 400), eachtile is evenly divisible by one (1), two (2), four (4), and five (5).The next larger whole number by which 400 is evenly divisible is eight(8). Therefore as the aircraft's altitude increases and the resolutionof the display may decrease, the next level of decreased resolutioninvolves subdividing the elevation posts within a tile into trianglesthat are eight elevation posts on a side. Assuming an initial groupingof 5, in preparation for a transition to the lower resolution, theprogram will direct the computer 14 to prepare a set of tiles in whichthe triangles are eight (8) elevation posts to a side in advance of theaircraft rising to an elevation where that reduced resolution would berequired. When the elevation threshold for a decrease in resolution iscrossed, the tiles for that level are available for culling to determinewhich should be included in the field of view. As the airplane continuesto rise, the process is repeated, next with tiles divided to include 10elevation posts, and the process is repeated. In descending the processis reversed.

The logic 58 for selecting and rendering tiles is shown schematically inFIG. 5. The first step 60 is to load the terrain tiles with all theirassociated terrain elevation data and geometric centers into memory 16.

Next, at 62 the computer acquires information about the aircraft'scurrent position (geographic location and altitude AGL) and heading. At64, the field of view angle and horizon range for the given altitude AGLare determined. Then, the tiles are examined one at a time by loading at66 a tile. If the center of the tile is within the field of view (angleand range) (steps 68 and 70) then the tile is passed to the graphicsprocessor at step 72 for rendering. The process is repeated until allthe tiles have been scanned and rendered or discarded.

During flight an aircraft's position is constantly changing. As aconsequence the field of view and horizon are also changing. The systemtakes this into account by updating the selection of tiles to bedisplayed and rendering a new image based on the new set of selectedtiles. This updating may occur, for example, once per second. (Thedisplay is refreshed 15 to 30 times per second, producing a smoothtransition on the display.) If the flight is level, new tiles are addedto the set of tiles beyond the horizon so that they will be availablefor the graphics processor when required. If the aircraft changesaltitude, the processor sets the number of terrain data points to begrouped together at a higher number (if the aircraft is ascending) or alower number (if the aircraft is descending). This has the effect ofincreasing the resolution as the aircraft approaches the ground. Oncethe group size has been reset, the process of selecting the terrainelevation data to be passed to the graphics processor for each groupingof terrain data points begins anew, and the display is updatedaccordingly.

Although the logic 28 and 58 (FIGS. 3 and 5) of the present invention isembodied in software or code executed by general purpose hardware asdiscussed above, as an alternative the logic 28 and 58 (FIGS. 3 and 5)may also be embodied in dedicated hardware or a combination ofsoftware/general purpose hardware and dedicated hardware. If embodied indedicated hardware, the logic 28 and 58 (FIGS. 3 and 5) can beimplemented as a circuit or state machine that employs any one of or acombination of a number of technologies. These technologies may include,but are not limited to, discrete logic circuits having logic gates forimplementing various logic functions upon an application of one or moredata signals, application specific integrated circuits havingappropriate logic gates, programmable gate arrays (PGA), fieldprogrammable gate arrays (FPGA), or other components, etc. Suchtechnologies are generally well known by those skilled in the art and,consequently, are not described in detail herein.

The block diagrams and/or flow charts of FIGS. 3 and 5 illustrate thearchitecture, functionality, and operation of an implementation of thelogic 28 and 58 (FIGS. 3 and 5). If embodied in software, each block mayrepresent a module, segment, or portion of code that comprises programinstructions to implement the specified logical function(s). The programinstructions may be embodied in the form of source code that compriseshuman-readable statements written in a programming language or machinecode that comprises numerical instructions recognizable by a suitableexecution system such as a processor in a computer system or othersystem. The machine code may be converted from the source code, etc. Ifembodied in hardware, each block may represent a circuit or a number ofinterconnected circuits to implement the specified logical function(s).

Although the block diagrams and/or flow charts of FIGS. 3 and 5 show aspecific order of execution, it is understood that the order ofexecution may differ from that which is depicted. For example, the orderof execution of two or more blocks may be scrambled relative to theorder shown. Also, two or more blocks shown in succession in FIGS. 3 and5 may be executed concurrently or with partial concurrence. In addition,any number of counters, state variables, warning semaphores, or messagesmight be added to the logical flow described herein, for purposes ofenhanced utility, accounting, performance measurement, or providingtroubleshooting aids, etc. It is understood that all such variations arewithin the scope of the present invention. Also, the block diagramsand/or flow charts of FIGS. 3 and 5 are relatively self-explanatory andare understood by those with ordinary skill in the art to the extentthat software and/or hardware can be created by one with ordinary skillin the art to carry out the various logical functions as describedherein.

Also, where the logic 28 and 58 (FIGS. 3 and 5) comprises software orcode, it can be embodied in any computer-readable medium for use by orin connection with an instruction execution system such as, for example,a processor in a computer system or other system. In this sense, thelogic may comprise, for example, statements including instructions anddeclarations that can be fetched from the computer-readable medium andexecuted by the instruction execution system. In the context of thepresent invention, a “computer-readable medium” can be any medium thatcan contain, store, or maintain the logic 28 and 58 (FIGS. 3 and 5) foruse by or in connection with the instruction execution system. Thecomputer readable medium can comprise any one of many physical mediasuch as, for example, electronic, magnetic, optical, electromagnetic,infrared, or semiconductor media. More specific examples of a suitablecomputer-readable medium would include, but are not limited to, magnetictapes, magnetic floppy diskettes, magnetic hard drives, or compactdiscs. Also, the computer-readable medium may be a random access memory(RAM) including, for example, static random access memory (SRAM) anddynamic random access memory (DRAM), or magnetic random access memory(MRAM). In addition, the computer-readable medium may be a read-onlymemory (ROM), a programmable read-only memory (PROM), an erasableprogrammable read-only memory (EPROM), an electrically erasableprogrammable read-only memory (EEPROM), or other type of memory device.

The present invention is particularly suited to be used in conjunctionwith the invention disclosed in a U.S. patent application entitledSYSTEM AND METHOD FOR PRODUCING FLIGHT PATHWAY, filed concurrentlyherewith; the entire disclosure of that application is incorporatedherein by reference.

Although the invention has been shown and described with respect tocertain preferred embodiments, equivalent alterations and modificationswill occur to others skilled in the art upon reading and understandingthis specification and the annexed drawings. In particular regard to thevarious functions performed by the above described integers (components,assemblies, devices, compositions, etc.), the terms (including areference to a “means”) used to describe such integers are intended tocorrespond, unless otherwise indicated, to any integer which performsthe specified function of the described integer (i.e., that isfunctionally equivalent), even though not structurally equivalent to thedisclosed structure which performs the function in the hereinillustrated exemplary embodiment or embodiments of the invention. Inaddition, while a particular feature of the invention may have beendescribed above with respect to only one of several illustratedembodiments, such feature may be combined with one or more otherfeatures of the other embodiments, as may be desired and advantageousfor any given or particular application.

1. A method of rendering an image of terrain for display in a cockpitdisplay, the method comprising: forming a set of tiles from raw terraindata, each tile including information concerning the elevation of pointson the ground; selecting a subset of tiles to be processed to render theimage; sending the subset of tiles to a graphics processor to render animage based on the information in each tile, each tile being associatedwith a specific area on the ground and including elevation data withrespect to the elevation of multiple locations within the specific area;selecting elevation data with respect to some but not all of themultiple locations within the specific area; forming a group ofelevation data; selecting representative elevation data from the group;arranging the data in a regular rectangular matrix; selecting a number(n) of data points to be representative data points; forming the groupinto an n sided polygon within the matrix; selecting a range of datapoints surrounding each vertex of the polygon such that every data pointwithin the polygon is within the selected range of at least one vertex;assigning to the data point at each vertex the highest value of any datapoint within the selected range of that vertex; and selecting the vertexdata points as the representative data.
 2. The method of claim 1 whereinthe step of selecting a subset of tiles includes the step of selectingthe number of data points representing information concerning theelevation of points on the ground from each tile to be rendered, whichnumber decreases with increasing aircraft altitude above ground level.3. The method of claim 2 wherein the step of forming a set of tilesincludes calculating the geometric center of each tile.
 4. The method ofclaim 3 wherein the step of selecting a subset of tiles includesidentifying tiles which have a geometric center within a preselectedangle of view.
 5. The method of claim 4 wherein the step of selecting asubset of tiles includes identifying tiles that have a geometric centerwithin a preselected distance from the aircraft.
 6. The method of claim4 wherein the step of identifying tiles which have a geometric centerwithin a preselected field of view includes the step of selecting tilesincluded within a selected angle measured with respect to the directionof motion of the aircraft and with respect to the altitude of theaircraft above ground level.
 7. The method of claim 1 wherein the numberof tiles in the selected subset of tiles increases as the aircraft'saltitude above ground level increases.
 8. The method of claim 1 whereinthe elevation data are grouped into triangle strips to form the groupsand the step of selecting some but not all of the elevation dataincludes selecting the data points at the vertices of each triangle. 9.A method of rendering dynamic real-time images of terrain during aflight of an aircraft for display in a cockpit display of the aircraft,the method comprising: forming a set of tiles from raw terrain data,each tile including multiple elevation data points associated within aspecific area on the ground; determining a position and a heading of theaircraft; selecting a subset of tiles to be processed to render theimages based on the aircraft's position and heading; selecting a highestelevation data point from among neighboring elevation data points fromeach selected tile us a representative elevation for all of theneighboring elevation data points; and sending the highest elevationdata point for each selected tile to a graphics processor and renderingeach selected tile using the elevation of the highest elevation datapoints in place of the neighboring elevation data points to render thedynamic real-time images to the cockpit display of the aircraft duringthe flight wherein terrain may appear higher than the specific area onthe ground.
 10. A program stored on a computer readable medium forperforming the method of claim
 9. 11. The method of claim 9 includingselecting a number of data points representing information concerningthe elevation of points on the ground from each tile to be rendered,wherein the number of data points changes with altitude of the aircraftabove ground level.
 12. The method of claim 9 wherein said forming a setof tiles includes calculating a geometric center for each tile.
 13. Themethod of claim 12 wherein said selecting a subset of tiles includesidentifying tiles that have a geometric center within a preselecteddistance from the aircraft during the flight.
 14. The method of claim 12wherein said selecting a subset of tiles includes identifying tileswhich have geometric centers within a preselected angle of view from theaircraft during the flight.
 15. The method of claim 14 wherein saididentifying tiles which have geometric centers within a preselectedfield of view includes selecting tiles included within a selected anglemeasured with respect to the direction, position, and altitude of theaircraft during the flight.
 16. A method of rendering dynamic real-timeimages of terrain during a flight of an aircraft for display in acockpit display of the aircraft, the method comprising: forming a set oftiles from raw terrain data, each tile including information concerningthe elevation of points on the ground; selecting a subset of tiles to beprocessed to render the image; sending the subset of tiles to a graphicsprocessor to render an image based on the information in each tile, eachtile being associated with a specific area on the ground and includingelevation data with respect to the elevation of multiple locationswithin the specific area; forming a group of elevation data; forming thegroup into a polygon; selecting a range of data points surrounding eachvertex of the polygon such that every data point within the polygon iswithin the selected range of at least one vertex; assigning to the datapoint at each vertex the highest value of any data point within theselected range of that vertex; selecting the vertex data points as therepresentative data; and sending the vertex data points so a graphicsprocessor to render the dynamic real-time images to the cockpit displayof the aircraft during flight.
 17. The method of claim 16 wherein theelevation data are grouped into triangle strips to form the groups andthe step of selecting some but not all of the elevation data includesselecting the data points at the vertices of each triangle.
 18. Themethod of claim 16 wherein the step of selecting a subset of tilesincludes the step of selecting the number of data points representinginformation concerning the elevation of points on the ground from eachtile to be rendered, which number decreases with increasing aircraftaltitude above ground level.
 19. The method of claim 16 wherein thenumber of tiles in the selected subset of tiles increases as theaircraft's altitude above ground level increases.
 20. The method ofclaim 16 wherein the step of forming a set of tiles includes calculatingthe geometric center of each tile.